lib: Document that flatpak_installation_update_fulls throws if up to date

This is not the ideal outcome, but it does avoid breaking
behaviour stability across versions.

Fixes #1336

Closes: #1337
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f